Clive Bealinga, Roman Martoňákb and Carla Moltenia, 
, 
Abstract
We compare the wurtzite (WZ) → rock salt (RS) transition mechanisms seen in a molecular dynamics simulation at elevated pressure and in a metadynamics simulation close to the experimental transition pressure. The mechanism seen in the molecular dynamics is characterized by a direct compression parallel to the direction, while in the metadynamics it is seen as a sliding of
layers parallel to the [0010]WZ direction. The h-MgO structure is seen as an intermediate in both cases, although it is only stable at the lower pressure. The RS structure produced by the direct compression and that which results from the sliding-layer mechanism make an angle of 15° with each other. Our metadynamics simulations along with other results suggest that sliding-layer mechanisms govern the transition close to the experimental transition pressure.
Keywords: Solid–solid transitions; Semiconductors; Molecular dynamics; Structure of bulk crystals; Crystallographic aspects of phase transformations; Pressure effects
Article Outline
Fig. 1. Evolution of the coordination of the 576-atom CdSe sample during the MD. The fractions of four- (black line), five- (red line), and sixfold (blue line) coordinated atoms are shown. The pressure is increased to P = 10.5 GPa at time t = 500 ps. At t =
508 ps five-coordination briefly takes over, when the boat- and chair-like structures in the c-direction collapse, forming a new Cd–Se bond. The five-coordination is short-lived however, and the transformation to six-coordinated RS is complete before t = 509 ps.Fig. 2. Evolution of the enthalpy at 300 K during the MD and for the structures obtained by quenching the room temperature structures to 0 K at the hydrostatic external pressure of 10.5 GPa.
Fig. 3. Mechanism of the WZ to RS transformation in the 576 atom CdSe sample raised to pressure of P = 10.5 GPa. The hexagonal WZ six-membered rings are compressed along the
Fig. 4. Coordination of the sample during the metadynamics. The fractions of four- (black line), five- (red line), and sixfold (blue line) coordinated atoms are shown. A change in coordination from fourfold to fivefold occurs at metastep
274, and the five-coordinated HS structure persists until metastep 290 at which point the sample transforms to the six-coordinated RS structure.Fig. 5. Evolution of the enthalpy at 300 K during the metadynamics and for the structures obtained by quenching the room temperature structures to 0 K at the hydrostatic external pressure of 2.5 GPa.
Fig. 6. Conversion of the HS structure to the RS structure during the metadynamics. (a) The sample during metastep 285. The transition to HS has already taken place. (b) The sample during metastep 288. An alternate pair of the
Fig. 7. Schematics illustrating four possible transition routes from WZ-type to RS-type CdSe. (a) M I:
Solid State Sciences Volume 12, Issue 2, February 2010, Pages 157-162 Morphology and dynamics of nanostructures and disordered systems via atomic-scale modelling |
